Answer:
2b² - 5ab
Explanation:
to find the missing side (??? ), subtract the sum of the 3 known sides from the perimeter, that is
9b² - 2ab + 12a² - (7b² - 2ab + 4a² - 4ab + 9ab + 8a² ) , simplify parenthesis
= 9b² - 2ab + 12a² - (7b² + 3ab + 12a²) ← distribute parenthesis by - 1
= 9b² - 2ab + 12a² - 7b² - 3ab - 12a² ← collect like terms
= 2b² - 5ab